1. Field of the Invention
This invention relates to fasteners for attaching a substrate to a bone, and more particularly to fasteners for anchoring soft tissue or bone plates to bone.
2. Prior Art
Both bioabsorbable and nonbioabsorbable bone fasteners adapted for attaching bone plates and soft tissue, such as, for example tendons and ligaments, to bone are known in the art. Such prior art fasteners include staples and tacks, screws, and rivets. Examples of staple type fasteners are illustrated in U.S. Pat. Nos. 4,454,875 and 4,570,623. Such bone staples generally include barbs on the cross-member which are useful for securely grasping a soft tissue such as a ligament and attaching it to bone.
The second type of fasteners include both screws and screw-washer combinations wherein a hole must be drilled in a bone for the purpose of receiving the screw. Screws may be fabricated from a surgically acceptable, biocompatible metal such as titanium, stainless steel or a cobalt-chromium alloy. Such metallic screws may be self-tapping. In bone fasteners comprising a screw-washer combination, the washer has spikes on one surface operable for grasping tissue, and a central aperture through which the screw is inserted into a bone. If the screw is fabricated from a bioabsorbable material, the hole in the bone must be tapped before the screw can be urged thereinto. Other ligament anchoring systems are disclosed in U.S. Pat. Nos. 4,927,421 and 4,870,957.
The staple and screw types of bone fastening devices possess several disadvantages. For example, staples, which are meant to be hammered into bone, must be made of a strong material, such as a metal. The use of staples is time consuming, traumatic and precludes the use of bioabsorbable polymers as suitable material for staple-type fasteners. As with staples, until such time as stronger and harder bioabsorbable materials become available, self-tapping screws must comprise a biocompatible metal. Both regular and self-tapping screw fasteners require a hole be drilled in the bone prior to use, If the screw comprises a non-metallic bioabsorbable material, the hole must also be tapped in order to receive the screw which requires an additional time consuming step.
In accordance with current art, metallic staples and screw fasteners are either permanently implanted within a bone, or a second surgical operation must be performed in order to remove them. In either case, implantation of metal fasteners does not allow for the gradual transfer of stress back to the bone/soft tissue junction as the healing proceeds. This, in turn, may slow down or impede the healing process. Furthermore, metal screws and staples may migrate from their original site of implantation over a period of time and lodge in a tissue causing pain. Permanently implanted metallic screw and staple fasteners can even migrate from the site of implantation to lodge within a joint, creating significant damage to articulator cartilage and other structures.
To overcome the disadvantages of the screw and staple types of bone fasteners, expandable rivets, both bioabsorbable and metallic, have been developed. Examples of such rivets are disclosed in U.S. Pat. Nos. 5,968,044; 5,911,721; and 5,725,529 to Nicholson et al., and U.S. Pat. No. 5,720,753 to Sander et al. Such rivets, which are either bioabsorbable or metallic, have the advantage that they may be inserted into an untapped hole, thereafter to be expanded, thereby reducing the time required for implantation of the rivets. Prior art rivets include an elongate body portion having an axial bore, an expanded head portion and an expandable leg portion. All of the prior art rivet-type bone fasteners include an expansion pin slidably mounted within the axial bore of the rivet. The bore and a portion of the expansion pin are configured such that movement of the expansion pin in an axial direction forces apart two or more legs on the rivet. The outer surface of the legs is adapted to engage the surrounding bone thereby preventing the rivet from backing out of the hole following implantation. The rivets include means for fastening a substrate to the rivet.
The expansion pins or functionally similar slidable elements used to expand the legs of the rivet bone fasteners, in accordance with the prior art, include a break-away portion which is not implanted in the bone with the rivet. On certain embodiments of the prior art rivets, tension must be applied to the expansion pin in order to expand the legs of the rivet. The tensile strength required to separate the traction portion of the expansion pin from the conical end portion may vary. Such tension may either pull the rivet out of the hole or cause the expansion pin to break prematurely. so that the security of the rivet within the bone is compromised.
Conversely, in other embodiments of the prior art rivets, the expansion pin is advanced into the rivet's axial channel in a distal direction (ie: deeper into the hole) to expand the legs. The expansion pin is urged into the axial channel by means of an insertion tool which is affixed to the expansion pin by frangible means. In the event that the insertion tool prematurely breaks away from the expansion pin, the rivet may not be securely anchored within the hole and prove difficult to remove. Accordingly, there remains a need for a fastener for securing tissue to bone which will have a predictable and sufficient initial anchorage strength to permit gradual load sharing to provide full repair and restoration of function of the tissue and bone. There exists a further need for a fastener device which is easily and rapidly attached to tissue and can be reliably anchored into bone which will not pull out of the bone or migrate from its original position following implantation.